The present invention relates to laser 3-dimensional printing (3DP) of metallic parts using a powder bed filled chamber, a well known “Additive Manufacturing” (AM) method, as well as laser 3DP using metallic powder spray, another popular AM method. The methods and apparatuses of the invention can also be applied in the areas of laser weld overlays and/or additive deposits produced with metallic powders.
The enumerated manufacturing technologies frequently utilize oxidation and/or H2O—H2-hydrocarbon gas sensitive metal powders, which include all types of alloy steels and superalloys, Ti, Al, Ni, Cu, precious metals, Co, Zr, Nb, Mo, W, Ta, Hf, Mg, and B alloys, as well as oxidation and/or H2O—H2-THC sensitive powder compositions resulting in composite parts containing metals, carbides, nitrides, aluminides, silicides, and/or borides. Consequently, reduction or elimination of sources of O2, H2O, CO2—CO, H2 and total hydrocarbons (THC) from the printing and the surroundings of the metal solidification area is desirable for improvement of process economics and product quality.
3DP methods of the prior art utilize nominally inert argon or helium, but overlook the issues of impurity content in these processing gases and in the printing apparatus, and fail to properly address atmosphere control in the powder feeding and printing surroundings. This results in reduced 3DP yields, lower productivity, and poor product quality.
Feed powders used in 3DP frequently include oxide films and inclusions, gas porosity, and adsorbed water. In addition (and depending on the powder making method used), many alloys, e.g. ferrous alloys, may contain substantial quantities of hydrogen (H) or nitrogen (N) dissolved in the solid matrix. When these powders are re-melted during typical 3DP operations, little time is available for the release of the contaminants before the molten pool of metal is solidified. A clean printing environment, e.g. a sweep stream of an ultra-pure Ar or He, can facilitate the removal of these contaminants. If the contaminants are not removed during the laser remelting and solidification, the product may contain oxide and nitride inclusions trapping H2 and N2 bubbles, as well as a solid matrix that's unnecessarily hardened and embrittled by the presence of undesired solutes. Entrapped bubbles deteriorate the surface finish and mechanical properties of the product. Non-metallic inclusions typically form with the most reactive alloy components, e.g. with Cr in stainless steels, which results in the removal of those alloying elements from the solid matrix and the subsequent loss of corrosion resistance. The presence of non-metallic inclusions in the product may, also, alter mechanical properties of the product in an undesired direction or promote microcracking during cooling.
Because oxygen, nitrogen, and other non-metallic elements are effective surface active agents even at very low concentration levels (on the order of parts per million), their precise control can influence the shape of molten metal pools formed and the formation of evaporative metal spatter taking place under laser beam. The melt pool and spatter are critical considerations for printing with increasingly popular, very fine metal powders which, on one hand, rapidly adsorb all environmental contaminants but, on the other hand, absorb the laser beam energy better, thus offering higher production rates, improved surface finish, and geometric resolution. It is expected that a range of recycled feed powders used in 3DP can be made to resemble the surface characteristics of the fine powders mentioned by precisely controlling the printing conditions, i.e. precise atmosphere control can improve 3DP operations using high loads of recycled powders. A “99.999%+” inert gas purity may be required to effectively reduce contaminant effects.
Hot isostatic pressing (HIP) and other heat treatments are frequently employed as post-processing steps on 3DP parts to correct some of the problems described. The need for and extent of post-processing steps could be reduced, though, if the 3DP conditions are improved. It is therefore the objective of this invention to improve the 3DP process and products by controlling the gas atmosphere used in the 3DP process.